1. Field of the Invention
This invention relates to a method and system for freeze protecting liquid NOx reductants and, more specifically, to a method and system for freeze protecting liquid NOx reductants used in vehicle applications.
2. Background Art
Diesel engines are typically more efficient than gasoline engines, with regard to fuel economy and emit less greenhouse gasses. However, diesel engines typically produce emissions containing higher concentrations of nitrogen oxides (NOx) compared to gasoline engines fitted with three-way catalysts. Future pollution control regulations may necessitate reducing the amount of NOx and particulate emissions produced by diesel engines.
One method that has been employed to remove NOx from diesel exhaust utilizes selective catalytic reduction (SCR) of NOx with a liquid nitrogen containing reductant, such as aqueous urea. NOx and the liquid reductant are brought into contact with a selective catalyst and catalytically converted into environmentally safe nitrogen and water. When a liquid reductant is used, the liquid reductant is typically injected directly into the exhaust pipe in front of a catalyst to effect reduction of NOx on the surface of the catalyst or in the catalyst itself.
One major disadvantage of liquid reductants is that freezing of the reductant may occur. The freezing temperature varies relative to the composition and concentration of the dissolved reductant. For example, solutions having a urea content of about 32.5% in water (eutectic), typically freeze at about 12xc2x0 F. (xe2x88x9211xc2x0 C.). As can be readily expected, liquid reductant freezing is particularly a problem for the use of diesel vehicles in cold-weather climates when a liquid reductant is employed to help meet emission standards for NOx. Also, expansion of the liquid reductant due to freezing can cause damage to the system components.
One approach to address the problem of supplying liquid reductant from a frozen reductant source uses heat to warm the liquid reductant above its freezing point. Heating methods have been developed using additional sources of energy, such as diesel fuel, to run a heater, or electrical supplemental heat to warm the liquid reductant, in cold weather conditions. Utilization of supplemental energy to warm the liquid reductant is disadvantageous because the supplemental energy requirement can result in an inefficient use of energy and decreased fuel economy. Supplemental heating by fuel sources is further disadvantageous since it requires a second fuel injection system thereby increasing costs and emissions. Another disadvantage is that the liquid reductant could be heated too much causing the liquid reductant to evaporate and therefore be ineffective.
It would be desirable to have a system that can employ liquid reductants for decreasing NOx emissions in cold weather climates without experiencing at least some of the above-mentioned disadvantages.
The present invention relates to a method and system for heating liquid reductant above its freezing temperature by utilizing existing heat generated by an engine under operating conditions, to enable use of the liquid reductant in cold weather conditions to reduce emissions of NOx, in conjunction with a catalyst, without decreasing overall fuel economy or overheating the liquid reductant.
This invention relates, more specifically to a method for operating an exhaust gas purification system. The method comprises directing fuel, returning from a high pressure fuel injection system, wherein the fuel becomes heated, to a reductant source, transferring heat from the fuel to the reductant to liquefy frozen reductant, and supplying the liquid reductant to an exhaust pipe at a location in front of a catalyst for purification of exhaust gas.
In a preferred embodiment of the invention, a heat exchanger with a reservoir may be the source of reductant liquefied by heat supplied to the heat exchanger by a high pressure fuel injection system through a return fuel line. In another preferred embodiment, a urea supply line contained within a return fuel line may be the source of reductant rapidly liquefied by heated fuel returning from a high pressure fuel injection system through a return fuel line.
This invention also relates to a system for operating an exhaust gas purification system. The system comprises a source of fuel, a first source of liquid reductant, and an exhaust pipe for discharging exhaust gas from the vehicle. The system further comprises a second source of liquid reductant that is disposed between the first source of liquid reductant and the exhaust pipe, a high pressure fuel injection system disposed between the fuel source and the second liquid reductant source, a first conduit fluidly connecting the fuel source with the high pressure fuel injection system, a second conduit fluidly connecting the high pressure injection system with the fuel source, and a third conduit fluidly connecting the first liquid reductant source with the exhaust pipe. The system further comprises a first high pressure fuel pump to deliver fuel from the fuel source through the high pressure fuel injection system, past the second liquid reductant source, returning to the fuel source. The compression of the fuel in the high pressure fuel injection system heats the fuel. The system further comprises a second pump to deliver liquid reductant from the second liquid reductant source to the exhaust pipe.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.